Sediment Accretion Rates for Natural Levee and Backswamp Riparian Forests in the Mobile-Tensaw Bottomlands, Alabama

SEDIMENT ACCRETION RATES FOR NATURAL LEVEE AND BACKSWAMP RIPARIAN FORESTS IN THE MOBILE-TENSAW BOTTOMLANDS, ALABAMA

Kathryn R. Kidd, Carolyn A. Copenheaver, and W. Michael Aust1

Abstract—Several methods to quantify sediment deposition patterns in riparian forested wetlands have been used during recent decades. In this study, we used a dendrogeomorphic technique with green ash (Fraxinus pennsylvanica) to estimate sediment accretion rates for two time periods (1881 to 2012 and 1987 to 2012) along a natural levee (35 m from river) and water tupelo-baldcypress (Nyssa aquatica-Taxodium distichum) backswamp (75 m from river) adjacent to the Tensaw River in southwestern Alabama. Sediment accretion rates were significantly higher for the 1987 to 2012 time period along the natural levee (p = 0.009; 1.6 cm yr-1) and backswamp (p = 0.032; 1.2 cm yr-1) than for the 1881 to 2012 period (0.4 and 0.5 cm yr-1). We further compared dendrogeomorphic sediment accretion rate estimates along the natural levee and backswamp to rates previously obtained using sediment pin and elevation survey methods at increased distances from the river (160 to 330 m) for the 1987 to 2012 time period. Regardless of method used we identified a negative trend in sediment accretion rates as distance from river increased and elevation decreased. Overall, this study demonstrates effective use of dendrogeomorphic techniques while minimizing site visits, compared to the other methods, to estimate sediment accretion rates across temporal and spatial scales.

INTRODUCTION are derived following a single site visit in which Development of methods to estimate the amount measurements are taken. of sediment trapped by riparian forested wetlands has followed a recognized need for empirical data Sediment accretion rates were previously estimated on sediment accretion in bottomland systems (Boto using sediment pin and elevation survey methods in a and Patrick 1979). Such methods include the use of water tupelo-baldcypress (Nyssa aquatica-Taxodium 137Cesium, feldspar clay marker horizons, sediment distichum) backswamp at distances of 160 to 330 m pins, elevation surveys, and dendrogeomorphic from the Tensaw River in southwestern Alabama (Aust techniques. Each of these methods has advantages and others 2012, McKee and others 2012). However, and disadvantages (USACOE 1993). Previous studies sediment pin and elevation based sediment accretion that have compared these methods have deemed each rates only represented a 24-year period (1986 to 2010) method valid for estimating sediment accretion rates in and these estimates were derived for only backswamp riparian forested wetlands (Cahoon and others 2000, locations. Therefore, the objectives of this study were Heimann and Roell, 2000, Kleiss 1996, USACOE 1993). to: 1) use the dendrogeomorphic technique to estimate sediment accretion rates along a natural levee (35 m The dendrogeomorphic technique uses the total from river) and backswamp (75 m from river) adjacent vertical depth of sediment accreted and tree age of to the Tensaw River for two time periods (1881 to 2012 an immediately adjacent tree to provide sediment and 1987 to 2012), and 2) compare dendrogeomorphic accretion rates over the life span of trees used in the rates with previous sediment pin and elevation survey estimation technique. This method was validated by estimates to determine the influence of distance from Hupp and Morris (1990) and has been used in several river on sediment accretion rates. studies to provide sediment accretion rate estimates for time periods spanning from 10 to over 100 years MATERIALS AND METHODS across varying topographic positions in bottomland Study Area forests (Heimann and Roell 2000, Hupp and Bazemore This study was conducted along the western bank 1993, Kleiss 1996, Phillips 2001). One major benefit of the Tensaw River within the Mobile-Tensaw River of the dendrogeomorphic technique is the estimates Delta. The site was located in Baldwin County, Alabama

1Graduate Research Assistant, Associate Professor, and Professor, Department of Forest Resources and Environmental Conservation, Virginia Tech, Blacksburg, VA 24061, respectively.

Citation for proceedings: Schweitzer, Callie J.; Clatterbuck, Wayne K.; Oswalt, Christopher M., eds. 2016. Proceedings of the 18th biennial southern silvicultural research conference. e–Gen. Tech. Rep. SRS–212. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 614 p.

PROCEEDINGS OF THE 18TH BIENNIAL SOUTHERN SILVICULTURAL RESEARCH CONFERENCE 13 approximately 50 km north of the Mobile Bay. Climate Laboratory Methods in this region is subtropical. On average, 1750 mm of Tree cores were air dried and glued to wooden mounts. precipitation is recorded annually with the greatest Cores were sanded with progressively finer sand precipitation recorded during July (NOAA, 2011). The paper until cellular structures became visible in the mean daily temperature is 19.2° C with the highest cross-sectional view under magnification. A tree-ring temperatures observed during June, July, and August. chronology was developed using cores extracted at DBH and ground level for the twenty green ash trees. The study site consisted of a natural levee and water Cores were visually cross-dated using the list method, tupelo-baldcypress backswamp (30º57’ N, 87º53’ in which narrow growth rings common among samples W). The natural levee was located parallel to the were identified and used as signature years to ensure riverbank and extended approximately 50 m inland proper alignment of dating (Yamaguchi 1991). Annual until transitioning into the backswamp. The overstory tree ring-widths were measured to the nearest 0.01 mm and midstory on the natural levee were characterized using the LinTab™ 5 RINNTECH® measuring system and by green ash (Fraxinus pennsylvanica), overcup oak TSAP-Win™ software (v. 4.69). Dated tree-ring width (Quercus lyrata), and water oak (Q. nigra) with fewer measurement values were verified to ensure quality of occurrences of swamp tupelo (Nyssa sylvatica var. visual cross-dating using COFECHA software (Holmes bicolor), black willow (Salix nigra), American elm (Ulmus 1983). Dating errors detected by COFECHA were americana), and hornbeam (Carpinus caroliniana) corrected. (Aust and Lea 1991). The backswamp was lower in elevation than the natural levee, which allows overbank Data Analysis floodwaters to pond and persist above the surface The two vertical depths measured at each sampled into summer months. Change in elevation within the tree were averaged. Average total sediment deposition backswamp was initially less than 15 cm (Aust and values were divided by the respective tree age to Lea 1991). Very poorly drained soils of the Levy (fine, estimate sediment accretion rates for the time period in mixed, superactive, acid, thermic Typic Hydraquents) which the tree was living. Average sediment accretion series characterized the backswamp (Aust and others rates were calculated for 1881 to 2012 and 1987 to 2012 2012). Thus, the backswamp was primarily composed time periods along the natural levee and backswamp. of naturally regenerated flood-tolerant water tupelo and Accretion rates associated with green ash trees baldcypress, but was also characterized by smaller established from 1881 to 1976 were used to calculate components of Carolina ash (Fraxinus caroliniana), the 1881 to 2012 estimates, while 1987 to 2012 sediment pumpkin ash (F. profunda), black willow, red maple accretion rates were calculated using trees established (Acer rubrum), and water elm (Planera aquatica) from 1987 to 1994. Dendrogeomorphic sediment (McKee and others 2012). Average basal area in the accretion rates were compared between the two backswamp was initially 75 m2 ha-1 with approximately time periods along the natural levee and backswamp. 85 percent comprised by water tupelo (Aust and Lea Comparisons were made using Wilcoxon-Mann- 1991). Buttonbush (Cephalanthus occidentalis) and Whitney tests within the NPAR1WAY procedure in SAS dwarf palmetto (Sabal minor) were common shrubs in 9.3 (SAS 2012). the understory for the backswamp and natural levee, respectively. Periodic sediment accretion data were available from previous periodic sediment pin (70 to 81 pins) Field Methods measurements and repeated elevation surveys between Vertical depth of accreted sediment was measured 1986 and 2010 at distances of 160, 250, and 330 m immediately adjacent to 20 green ash trees along from the river (Aust and others 1991, Aust and others transects parallel to the river. Ten co-dominant 1997, Aust and others 2012, Gellerstedt and Aust 2004, trees were sampled on the natural levee and ten McKee and others 2012, Warren 2001). Sediment pin along the front edge of the backswamp. Using a measurements and elevation surveys were conducted dendrogeomorphic technique previously validated by across the backswamp on 20 x 20 m grids. To identify Hupp and Morris (1990), the original lateral roots were the influence of distance from river on sediment located on two sides of each ash tree. Once roots accretion rates, differences among dendrogeomorphic were located, the vertical distance between the lateral rate estimates at 35 and 75 m and sediment pin roots and current ground line was measured to the measurements at 160, 250, and 330 m were evaluated nearest 0.25 cm. Depths were measured 0.5 m away using a Kruskal-Wallis test. If significant, pairwise from the tree base to avoid interference of the base on Wilcoxon-Mann-Whitney tests were conducted between deposition totals. To determine the time period in which the distances. Differences among all five distances the vertical sediment accumulated, tree cores were using the dendrogeomorphic and elevation survey data extracted below ground level and at DBH (diameter at were evaluated in an identical method. Exact methods breast height) for each tree using an increment borer.

14 SOIL AND SITE RELATIONSHIPS were specified for all nonparametric tests. All statistical attributed differences to changes in upstream land use analyses were performed at a significance-level and channelization (Heimann and Roell 2000, Hupp α = 0.05. and Morris 1990, Kleiss 1996). While upstream land use could have potentially impacted sediment accretion RESULTS AND DISCUSSION rates in this study, the localized impacts of the adjacent Dendrogeomorphic Sediment Accretion Rates harvesting disturbance are most likely the reason for the Mean sediment accretion rates on the natural levee differences. Another potential influence on the longer were significantly higher (124 percent, p = 0.009) for term (1881 to 2012) sediment accretion rates is erosion. the 1987 to 2012 time period (1.6 ± 0.2 cm yr-1) than Differences between gross and net sediment accretion the 1881 to 2012 time period (0.4 ± 0.0 cm yr-1) (fig. 1). rates have been reported in wetlands adjacent to the Similarly, sediment accretion rates were significantly Olentangy River in Ohio (Mitsch and others 2014). In higher (p = 0.032, 72 percent) along the backswamp our study, sampled areas were in direct contact with for the 1987 to 2012 time period (1.2 ± 0.2 cm yr-1) than overbank floodwaters which could have potentially the 1881 to 2012 time period (0.5 ± 0.1 cm yr-1) (fig. 1). eroded exposed portions of the natural levee and Differences in sediment accretion rates between the backswamp through time. two time periods were attributed to altered sediment Sediment Accretion Rates With Increased Distance trapping efficiency due to changes in the dynamics of ground flora and understory vegetation following Mean sediment accretion rates decreased as the an adjacent forest harvest disturbance in 1986. This distance from the river increased from 35 m to 330 m for disturbance likely increased levels of light penetration the 1987 to 2012 time period (fig. 2). Dendrogeomorphic and thus, initially resulted in increased ground flora and sediment accretion rate estimates for the natural understory vegetation densities (Aust and others 1997). levee and backswamp were higher than in the water An initial increase in low, dense vegetation is likely the tupelo-baldcypress backswamp stand at distances main reason for higher sediment accretion rates during of 160 to 330 m from the river. A Kruskal-Wallis test the 1987 to 2012 time period. Similar increases in indicated sediment accretion rates estimated from sediment trapping efficiency with increased vegetation dendrogeomorphic and sediment pins were significantly have been noted following altered vegetation dynamics different (p < 0.001) for at least one distance (35, 75, following harvesting disturbances (Aust and others 160, 250, and 330 m from the river). Pairwise Wilcoxon- 2006, Perison and others 1997). Other studies that have Mann-Whitney tests confirmed dendrogeomorphic -1 identified temporal patterns in sediment accretion have estimates at 35 m (1.6 ± 0.2 cm yr ; p = 0.034) and

3.0 Estimate period ) -1 1881 to 2012 2.5 1987 to 2012 a 2.0 a

1.5

b 1.0

Sediment accretion rates (cm year 0.5

0.0 Natural levee (35 m) Backswamp (75 m)

Topographic position and distance from river

Figure 1—Sediment accretion rates (cm yr-1) estimated using a dendrogeomorphic technique on green ash along a natural levee (35 m from river) and backswamp (75 m from river) adjacent to the Tensaw River. Estimates were derived for different time periods: 1881 to 2012 (tree ages ranged from 60 to 131 years) and 1987 to 2012 (tree ages ranged from 20 to 25 years). Mean estimates are indicated by closed markers. Different letters indicate estimates are significantly at α = 0.05 based on Wilcoxon-Mann-Whitney tests.

PROCEEDINGS OF THE 18TH BIENNIAL SOUTHERN SILVICULTURAL RESEARCH CONFERENCE 15 2.0 Dendrogeomorphic a Distance from river (m) )

-1 Sediment pins

1.5 ab Elevation surveys

abc

1.0 bc bc bc c c

0.5 Sediment accretion rate (cm yr

0.0 35 75 160 250 330 Distance from river (m)

Figure 2—Mean sediment accretion rates (cm yr-1) for the 1987 to 2012 time period as distance from the Tensaw River increased from the natural levee (35 m) across the mature tupelo-cypress backswamp (75 to 330 m) based on dendrogeomorphic estimates, previous sediment pin measurements, and elevation surveys. Different letters indicate estimates are significantly at α = 0.05 based on Wilcoxon-Mann-Whitney tests.

75 m (1.2 ± 0.2 cm yr-1; p = 0.036) were significantly to differences among studies in site characteristics and higher than sediment pin accretion rate estimates at connectivity to sediment-laden waters (Hupp and others 160 m (0.8 ± 0.0 cm yr-1). Dendrogeomorphic sediment 2015). Decreased sediment accretion rates at increased accretion rates at 35 m were significantly different distance in our study may have been influenced by the from sediment pin estimates at 250 m (0.9 ± 0.0 cm size of particles deposited. Larger particles generally yr-1; p = 0.034) and 330 m (0.8 ± 0.0 cm yr-1; p = 0.047). fall out of suspension first, as overbank floodwater Analysis of the dendrogeomorphic and elevation survey velocity decreases (Boto and Patrick 1979). Therefore, estimates among the same five distances indicated particle size could have influenced total vertical values for at least one distance were significantly deposition values with change in distance from the different (p = 0.043). Additional pairwise comparisons river. Along the Mississippi River, the percentage of revealed dendrogeomorphic estimates at 35 m were sand (0.063 to 2 mm) particles that were deposited on significantly different from elevation survey estimates the natural levee (68 percent) and the levee backslope at 250 (0.6 ± 0.1 cm yr-1; p = 0.047) and 330 m (0.7 ± 0.1 (47 percent) were greater than in the backswamp (3 cm yr-1; p = 0.047). Sediment accretion rates estimated percent) (Kesel and others 1974). Smaller silt (0.002 to from the dendrogeomorphic technique at 35 and 75 m < 0.063 mm) and clay (< 0.002 mm) particles made up were higher than the elevation survey estimates at 160 particles in the backswamp than along the natural levee (0.8 ± 0.4 cm yr-1), but were not statistically different and levee backslope (Kesel and others 1974). Similar (p = 0.250 for both distances) due to an elevated findings in the distribution of particles by size have been value at the 160 m distance (fig. 2). As expected, reported along transects extending from natural levees dendrogeomorphic sediment accretion rate estimates into backswamps in the Atchafalaya Basin in Louisiana at 75 m were significantly higher than elevation survey (Hupp and others 2008) and in floodplains along Long based estimates at 250 m (p = 0.036) but not at 330 m Brank Creek in Missouri (Heimann and Roell 2000). (p = 0.140). CONCLUSION In our study, as distance from the Tensaw River Riparian forests have the capacity to trap and store increased, sediment accretion rates for the 1987 to significant amounts of sediment from adjacent 2012 time period decreased. At our study site, elevation waterways. This study used a dendrogeomorphic also decreased with increased distance from the river. technique, which minimized visits, to illustrate temporal Trends in sediment deposition with changes in distance patterns in sediment deposition. Further, this study from river and elevation both agree (Asselman and identified spatial patterns of sediment deposition; Middlekoop 1995, Johnston and others 1984, Kesel and specifically, decreased sediment accretion rates with others 1974) and disagree (Hupp and Bazemore 1993, increased distance from river and decreased elevation. Hupp and Morris 1990, Kleiss 1996) with our results due Trends in repeated sediment deposition have numerous

16 SOIL AND SITE RELATIONSHIPS implications on successional pathways in riparian Holmes, R.L. 1983. Computer-assisted quality control in tree- forested wetlands. Overall, the dendrogeomorphic ring dating and measurement. Tree-Ring Bulletin. 43: 69-78. technique was effective in supplementing sediment Hupp, C.R.; Bazemore, D.E. 1993. Temporal and spatial patterns accretion rate estimates derived from sediment pin and of wetland sedimentation, West Tennessee. Journal of Hydrology. 141(1-4): 179-196. elevation survey methods to quantify the amount of sediment being trapped through time and space. Hupp, C.R.; Demas, C.R.; Kroes, D.E. [and others]. 2008. Recent sedimentation patterns within the central Atchafalaya Basin, ACKNOWLEDGMENTS Louisiana. Wetlands. 28(1): 125-140. This study was funded by the United States Department Hupp, C.R.; Morris, E.E. 1990. A dendrogeomorphic approach to measurement of sedimentation in a forested wetland, Black of Agriculture, National Needs Fellowship Grant Swamp, Arkansas. Wetlands. 10(1): 107-124. 2010-38420-21851. Previous sediment accretion data Hupp, C.R.; Schenk, E.R.; Kroes, D.E. [and others]. 2015. collections were supported by the National Council for Patterns of floodplain sediment deposition along the Air and Stream Improvement, Inc. We would also like regulated lower Roanoke River, North Carolina: annual, to thank the Alabama Department of Conservation and decadal, centennial scales. Geomorphology. 228: 666-680 Natural Resources and the Forever Wild Land Trust for Johnston, C.A.; Bubenzer, G.D.; Lee, G.B. [and others]. 1984. granting permission to continue this long-term study. Nutrient trapping by sediment deposition in a seasonally Additional field assistance was provided by John flooded lakeside wetland. Journal of Environmental Quality. Peterson and Greg Ward. 13(2): 283-290. Kesel, R.H.; Dunne, K.C.; McDonald, R.C. [and others]. 1974. LITERATURE CITED Lateral erosion and overbank deposition on the Mississippi River in Louisiana caused by 1973 flooding. Geology. Asselman, N.E.; Middelkoop, H. 1995. Floodplain sedimentation: 2(9): 461-464. quantities, patterns and processes. Earth Surface Processes and Landforms. 20(6): 481-499. Kleiss, B.A. 1996. Sediment retention in a bottomland hardwood wetland in eastern Arkansas. Wetlands. 16(3): 321-333. Aust, W.M.; Fristoe, T.C.; Gellerstedt, P.A. [and others]. 2006. Long-term effects of helicopter and ground-based skidding McKee, S.E.; Aust, W.M.; Seiler, J.R. [and others]. 2012. on site properties and stand growth in a tupelo-cypress Long-term site productivity of a tupelo-cypress swamp 24 wetland. Forest Ecology and Management. 226(1-3): 72-79. years after harvesting disturbances. Forest Ecology and Management. 265: 172-180. Aust, W.M.; Lea, R. 1991. Soil temperature and organic matter in a disturbed forested wetland. Soil Science Society of America Mitsch, W.J.; Nedrich, S.M.; Harter, S.K. [and others]. 2014. Journal. 55(6): 1741-1746. Sedimentation in created freshwater riverine wetlands: 15 years of succession and contrasts of methods. Ecological Aust, W.M.; Lea, R.; Gregory, J.D. 1991. Removal of floodwater Engineering. 72: 25-34. sediments by a clearcut tupelo-cypress wetland. Water Resources Bulletin. 27(1): 111-116. NOAA [National Oceanic and Atmospheric Administration]. 2011. Summary of Monthly Normals, 1981-2010, National Climatic Aust, W.M; McKee, S.E.; Seiler, J.R.; Strahm, B.D. 2012. Long- Data Center. http://www.ncdc.noaa.gov/data-access/ term sediment accretion in bottomland hardwoods following land-based-station-data/land-based-datasets/climate- timber harvest disturbances in the Mobile-Tensaw River Delta, normals/1981-2010-normals-data [Date accessed: February Alabama, USA. Wetlands. 32(5): 871-884. 22, 2015]. Aust, W.M.; Schoenholtz, S.H.; Zaebst, T.W.; Szabo, B.A. 1997. Perison, D.; Phelps, J.; Pavel, C.; Kellison, R. 1997. The effects Recovery status of tupelo-cypress wetland seven years after of timber harvest in a South Carolina blackwater bottomland. disturbance: silvicultural implications. Forest Ecology and Forest Ecology and Management. 90(2-3): 171-185. Management. 90(2-3): 161-169. Phillips, J.D. 2001. Sedimentation in bottomland hardwoods Boto, K.G.; Patrick, W.H. 1979. Role of wetlands in the removal downstream of an east Texas dam. Environmental Geology. of suspended sediments. In: Greeson, P.E.; Clark, J.R.; Clark, 40: 860-868. J.E., eds. Wetland functions and values: the state of our understanding. In: Proceedings of national symposium on SAS. 2012. SAS/STAT 9.3 User’s Guide: The NPAR1WAY wetlands, Lake Buena Vista, FL. TPS 79-2. Minneapolis, MN: Procedure. Cary, NC: SAS Institute, Inc. American Water Resources Association: 479-489. USACOE [U.S. Army Corps of Engineers]. 1993.Methods of Cahoon, D.R.; Marin, P.E.; Black, B.K.; Lynch, J.C. 2000. A measuring sedimentation rates in bottomland hardwoods. method for measuring vertical accretion, elevation, and WRP Technical Note SD-CP-4.1. Vicksburg, MS: U.S. Army compaction of soft shallow-water sediments. Journal of Corps of Engineers Waterways Experiment Station. 7p. Sedimentary Research. 70(5): 1250-1253. Warren, S.E. 2001. Sedimentation in a tupelo-baldcypress Gellerstedt, P.A.; Aust, W.M. 2004. Timber harvesting effects forested wetland 12 years following harvest disturbance. after 16 years in a tupelo-cypress swamp. In: Connor, K.F., ed. Blacksburg, VA: Virginia Polytechnic and State University. Proceedings of the 12th biennial southern silvicultural research 72 p. M.S. thesis. conference. Gen. Tech. Rep. SRS-71. Asheville, NC: U.S. Yamaguchi, D.K. 1991. A simple method for cross-dating Department of Agriculture Forest Service, Southern Research increment cores from living trees. Canadian Journal of Forest Station. 4 p. Research. 21(3): 414-416. Heimann, D.C.; Roell, M.J. 2000. Sediment loads and accumulation in a small riparian wetland system in northern Missouri. Wetlands. 20(2): 219-231.

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